The present invention relates to a high voltage semiconductor device, and more particularly, to a high voltage semiconductor device having an additional region next to a gate electrode, which may be a high dielectric constant (high-k) dielectric such as nitride or a conductor such as polysilicon and a method for manufacturing such a high voltage semiconductor device having an additional nitride or polysilicon region.
Metal oxide semiconductor (MOS) transistor devices are generally known in the art. A metal oxide semiconductor field effect transistor (MOSFET) is a widely used type of field effect transistor (FET). MOSFETs can either be n-channel (NMOS) or p-channel (PMOS) transistors, and may be utilized for high power applications. When a voltage is applied between a gate and source terminal of the MOS, an electric field generated penetrates through an oxide layer and creates a so-called “inversion channel” in a channel underneath the gate. The inversion channel is of the same type, i.e., p-type or n-type, as the source and drain, so it provides a conduit through which current can pass.
In NMOS transistors, the silicon channel between the source and the drain of the device is p-type silicon. When a voltage larger than a threshold voltage is placed on the gate electrode, electrons in the p-type material are inverted and then “conduct” through the channel turning the device “ON.” Conversely, when a voltage less than a threshold voltage is applied to the gate, the device turns “OFF.” In PMOS transistors, the silicon channel between the source and the drain of the device is n-type silicon. In a CMOS, no power flows until the transistors switch.
FIG. 1A shows a typical prior art symmetric high voltage metal oxide semiconductor (MOS) device 100 having an oxide fill 111. The MOS device 100 includes a semiconductor substrate 102, a source 104, a drain 112, a gate 125 and a channel region 105. The source 104 has a metal or polysilicon contact 104a, and the drain 112 also has a metal or polysilicon contact 112a. The oxide fill 111 surrounds at least the sides of the gate 125 and covers at least a portion of a main surface 102a of the semiconductor substrate 102. The MOS device 100 includes lightly doped regions 126, 128 in the semiconductor substrate 102 between the gate 125 and the source 104 and the drain 112, respectively. Similarly, FIG. 1B shows a typical prior art asymmetric high voltage MOS device 100 having similar attributes.
FIG. 2 is a graph of drain current Id versus drain voltage Vd for the prior art high voltage MOS device 100 demonstrating “quasi-saturation effect” which occurs with increasing gate voltage. The drain current of the high voltage MOS device 100 saturates with increasing gate voltage Vg due to carriers in the n− region 128 reaching saturation velocity which thereby determines saturation current. By increasing n− dosage, quasi-saturation currents can be enhanced, but this solution undesirably decreases the breakdown voltage of a high voltage MOS device 100.
FIG. 3 shows gate fringing electrical field lines of the prior art high voltage MOS device 100. The existence of the fringing electrical field makes the additional gate voltage dependence of the carrier concentration in the n− region 128. FIG. 4 is a graph of electron concentration versus position in a channel for the prior art high voltage MOS device 100 demonstrating increasing carrier concentrations with increasing gate voltage Vg. Carrier concentrations in the lightly doped regions 126, 128 increase with increasing gate voltage Vg because of the gate fringing electric field. That is why there is still minor gate voltage dependence on drain current Id even though the high voltage MOS device enters quasi-saturation region.
It is desirable to provide a high voltage MOS device having enhanced gate fringing electrical fields and increased quasi-saturation current as compared to conventional high voltage MOS devices. This can be achieved by providing a high voltage MOS device having an additional region next to its gate electrode, which may be a high-k dielectric such as nitride or a conductor such as polysilicon.